Frequency diversity telegraph system



H. A. RHODES FREQUENCY DIVERSIIY TELEGRAPH SYSTEM Flled June 30, 1945 2 Sheets-Sheet 2 INVENToR E/aOa/e Patented Apr. 2.1946

\ FBEUENCY DIVERSITY TELEGBAPH SYSTEM Harold Anson hodes, lloomileld. N. J., assigner to American Telephone and Telegraph Company, a corporation of New York Application June 3o, 1943. serial No. 492,788

(ci. 17a-ss) 5 Claims.

'I'his invention relates generally to signaling systems, and more particularly to telegraph systems. lates to two-tone 'telegraph systems, as well-as to frequency diversity telegraph systems employing either wire line, radioor other transmission media.

Telegraph signals are often transmitted either by interrupting the current of a battery or other source of direct current in accordance with signals or by reversing the polarity of the current of the source. In 4both of these types of transmission the transmitting medium may be such that the signals become greatly attenuated and sometimes lost. It has been proposed to overcome the effect of the attenuation characteristic of the transmitting medium `by employing a two-tone method for conveying the signals. In the two-tone method current of one frequency is produced for a marking signal and current of a different frequency for a spacing signal. It has also been proposed to employ a limiter at the receiver to substantially reduce the eect of the attenuation characteristic of the transmitting medium. However, when one of the currents, such as the marking current, becomes 'attenuated or faded, the limiter will introduce noise and eiitraneous currents and transmit these undesired currents to the telegraph receiving urelay. Hence the signals may become distorted and perhaps rendered unintelligible.

In accordance with this invention it is proposed to transmit telegraph signals by a modified frequency diversity two-tone method. By this method currents of two discrete frequencies will be produced for each marking signal and currents of two still other discrete frequencies for each spacing signal. One of the currents of each pair will be produced by modulation with current of a common oscillator and will thereby be positioned in a somewhat higher position in the frequency spectrum. Hence each marking signal will be conveyed by a low frequency current and by a high frequency current resulting from modulation and each spacing signal will likewise be conveyed by a low frequency current and by a high frequency current resulting from modulation. 'I'hus the marking and spacing signals of any one telegraph channel will be transmitted by means of currents of `four diierent frequencies. At the receiving station the two higher frequencies resulting from modulation at the transmitter will be reduced in the frequency spectrum by demodulation with current of a common oscillator. The two currents of different Still more particularly, this invention resignals. The two currents corresponding to each marking signal will then be combined and detected to actuate a common telegraph receiving relay and likewise the two currents corresponding to each spacing signal will be thereafter combined and detectedvto actuate the same telegraph receiving relay. 4Thus the telegraph receiving relay will reproduce the marking and spacing-signals originating at the transmitting station. a

In the specific embodiment to be described each marked signal will cause the production of current of one frequency and each spacing signal will cause the production of current of a dierent frequency. 'I'hese two currents will be transmitted over one path, while over another path these same currents will be modulated upon current of a different frequency to produce the two higher frequencies corresponding to the same signais, as already noted. Thus the transmitter of the system will produce current of one frequency and a modulation product of a higher frequency in accordance with each marking signal, and current of a different frequency and a modulation product of a higher frequency in accordance with each spacing signal, as mentioned above. four of these currents will be transmitted to the receiving station where the two lower frequencies (which correspond to the marking and spacing signals respectively) will traverse one path and l the two higher frequencies (which also correspond to the same marking and spacing signals respectively) will traverse another path which includes demodulating apparatus for reducing the higher frequencies to lower positions in the frequency spectrum. The currents corresponding to the marking signals traversing both paths will be combined, amplified, then limited in amplitude and detected. The remaining currents corresponding to the spacing signals will also be combined, amplified and limited in amplitude and thereafter detected.

This invention will be better understood from the more detailed description hereinafter following when read in connection with the accomgenerally the apparatus required for producing currents .of two different frequencies in accordance with marking and spacing signals at the transmitting station.

Referring to Fig. 1 of the drawings, the refer- 'ence character on represents a low frequency oscillation generator which supplies current to a plurality of. devices 011-023, all of which may 4produce different harmonics of the current of the oscillator On. If the oscillator Oo generates current of, for example, 85 cycles per second, the devices O11 and O12, may produce the two harmonics or tones of, for example, 425 and 595 cycles per second respectively, for transmitting the marking and spacing signals of one of the telegraph channels. The otherldevices Oia-Osa may similarly produce still higher harmonics of the same base frequency of oscillator Oo for transmitting the marking and spacing signals of other telegraph channels. To simplify'the description, all of these devices 011-023 will be referred to hereafter as oscillators.

The apparatus 'for controlling the oscillators O11 and O12 for one of the channels is shown in Fig. 3. The key `K1 may be closed to operate the relays R1 andl Rz. When the key K1 is closed in accordance with a marking signal, the. armatures of both relays R1 and Rz will move to their marking contacts M so that the oscillator O11 will freely transmit its current, while the current of oscillator O12 will be'suppressed by the shunt circuit established by the armature and contact M of relay Ra around this oscillator. When key K1 is opened in accordance with a spacing signal, the relay armatures will close their spacing con'- tacts S, in which case the oscillator 011 will freely transmit current while the current 4of oscillator O11 will be suppressed. Similarly the oscillators 01s and O14 will be controlled by the marking or spacing signals of another telegraph channel, oscillators 01s and 01e will be similarly controlled by the marking or spacing signals of a third telegraph channel, and so on. The oscillator O2: may be used for supplying current for an order circuit, but in this case the current of oscillator 02s may be interrupted in accordance with marking and spacing signals.

All of the oscillators 011 to Oi: are connected to telegraph sending circuits designated SSu to SS23- and to filters F11 and F23, respectively, each pair of telegraph sending circuits SS11 to SSzzvbeing of the type shown in Fig. 3, and designed to freely transmit only the currents of the 'associated oscillators O11 to O22, respectively, as already mentioned with regard to Fig. 3. The sending circuit SScs may be of a simple type employing a key m for interrupting the current of oscillator O23. The outputs of all of the filters F11 to F23 are connected to a common bus CB1, which receives all of the currents of both marking and spacing signals of all of thetelegraph channels. 'I'he common bus CB1 is connected through a line L1 to two paths, the upper of which extends to a low pass filter F31 and the other includes an amplifier A1 and a modulator MD and extends to a high pass lter F32. The modulator MD is supplied with current from another oscillator Oso, which is also connected to the base frequency oscillator Oo, the oscillator asoman Oan producing a still higher harmonic of the current of oscillator Oo of. for example, 5,270 cycles. The filter F31 is of the low pass type-and is employed to transmit the currents of oscillators O11 to On, extending over a range of from 425 to 2,465 cycles, for example, while the lter Fas is of the high pass type and willtransmit thesel same currents raised in the frequency spectrum as a result vof modulation by the modulator MD. The currents transmitted by filter Fai may'extend from, for example, 2,805 to 4,845 cycles. The currents transmitted by both filters F31 an`d F32 will be amplied by amplifier A: and then radiated through space by a radio transmitter RT, which is supplied witha high or radio frequency current generated by an oscillator y0:11.

Thus the marking and spacing signals corresponding to the first of the telegraph channels will be transmitted by oscillators O11 and O12 in accordance with the two-tone method, and these currents will be transmitted by the low pass filter Fn, while these same currents will be raised in the frequency spectrum by modulator MD for transmission through the high" pass filter F32. If the oscillators 0 11 and O12 produce currents of 425 and 595 cycles, as already stated, the modulation currents corresponding to the same signals produced by modulator MD and transmitted by lter Fan will have frequencies of 4,845 and 4,675 cycles. These four -currents are employed for frequency diversity to convey the marking and spacing signals of but one telegraph channel, the currents of 425 and 4,845 cycles corresponding to a marking signal and the currents of 595 and 4,675 cycles corresponding tota spacing signal.

The signals received by the radio receiver RR of Fig. 2 are demodulated by a locally produced current emanating from oscillator 04o. The demodulated currents are then amplified by ampliyfier Aa and then divided into two bands, one of which is selected and transmitted by the low pass vselectively transmitting currents having the frequencies of oscillators 011 to O23, respectively'.

The output circuit of filter F411 is connected to a demodulator DM which is supplied with the current of an oscillator 041 for demodulating the received currents in the higher portion of the range so as to reduce these currents to lower frequencies. The lower frequencies resulting from demodulation extend from 765 to 2805 cycles. These currents are amplified by amplifier A4 and are then impressed upon a common bus CBs. The common bus CB; is bridged by filters F61 to Frs, which correspond to filters F11 to Fas and their respective oscillators O11 to 02s at the transmitting station. The output circuits of filters F41 and F42 are connected to each other and supply the low frequency marking and spacing currents alternately to a hybrid network HN for telegraph channel l. The filters F61 and Fea are also connected to each other and supply the higher frequency marking and spacing currents to the same and connected to an amplifier As.

frequencies of 425 and 595 cycles while filters Fn and Fez may transmit, for example, currents of '765 and 925 cycles. combines these currents as received, the combined currents are then amplified by a common ampliiler As, and the amplified currents are transmitted through a common current limiter CL.

The output circuit of -limiter CL is bridged by four filters designated Fai, Far. For and Fn. The i'llters F111` and F91 may be the same as the filters F41 and F61, respectively, and filters Fan and-F92 may be the same as filters F42 and Fez. The filters Fei and F91 will transmit currents of two different frequencies corresponding to a single marking signal, and filters Fs: and F92 will transmit currents of. two different frequencies corresponding to a single spacing signal. 'I'he filters F51 and F91 are connected to each other and to a detector D1 which detects the currents corresponding to a marking signal. The filters Fan and Fez are connected to each other and to a detector D2 which detects the currents corresponding to a spacing The hybrid coil network HN signal. The relay Rs connected to both detectors may transmit the signals to a teletypewriter TTY,

a recorder or any other signal indicating appan ratus.

Although the amplifier A5, the current limiter CL, filters Fai, Fez and F91 and Fez, detectors D1 and D2 and' relay R3 are shown common to but one of the telegraph channels, it will be understood that similar apparatus (not shown) is common to each of the other telegraph channels and operates in the same manner. The o'rder circuit marking and spacing signals-which are of the The arrangement of the invention embodies a frequency diversity system in which two currents of different frequencies, such as 425 cycles and '165 cycles perfsecond, transmitted in accordance with each marking signal, and two currents of still other frequencies, such as 595 cycles and 935 cycles per second, transmitted in accordance with each spacing signal, are impressed upon the common limiterlCL. The limiter GL accentuates the stronger of the two marking currents while effectively suppressing the weaker marking current and, moreover, the limitery CL accentuates the stronger of the two spacing currents while effectively suppressing the weaker spacing current. Thus, if the amplitude of the 765 cycle marking current exceeds the amplitude of the 425 cycle marking current, the stronger '165 cycle marking current will be effectively selected by thel limiter CL while the weaker 425 cycle marking current will be effectively suppressed along with any extraneous noise present. Similarly, the stronger spacing current will be effectively selected bythe limiter CL and the weaker spacing current and extraneous noise will be effectively suppressed. The filters Fa1, Fez, F91 and Fez, connected in parallel to the limiter CL, act to transmit the stronger marking and spacing currents, whichever they may be, to detectors D1 and Dz. If the 765 cycle marking currentis the stronger, it will be transmitted by filter F111 and detected by detector D1. Likewise, if the 595 cycle yspacing current is the stronger, it will be transmitted by filter Fez and detected by deteci301' D2.

It is also noted that currents of two different "frequencies are derived from one of the oscillainterrupted type-are vselected by lters F53 and F13, the output circuits of which are combined These signals are therefore amplified and are then caused to operate a loud speaker LS.

It will be observed that but one current limiter such as CL is used in each telegraph channel to act on all of the currents corresponding to the marking and spacing signals in any one channel.

As is well understood, the making currents will be interspersed with the spacing signalsso that in general either the marking currents or the spacing currents always will be present and acted on by limiter CL. When one of the two marking currents, for example, is reduced in amplitude or becomes otherwise faded due to the attenuation characteristic of the transmitting medium, the other marking current will be effectively selected by the limiter CL to transmit a strong marking signal. The appropriate filter connected to the output circuit of the limiter will pass a strong marking signal to the detecor Di to actuate the relay R3. The limiter CL will perform in the same' manner When one of the spacing currents of the channel becomes faded, the limiter acting to produce a strong spacing signal. Thus when any of the various marking or spacing currents are appreciably reduced in amplitude due to the attenuation characteristic of the transmitting` medium, the limiter of the channel will effectively overcome this condition and the signals will nevertheless be reproduced accurately. The effect 0f noise and extraneous currents will be overcome in each telegraph channel by a single limiter.

lspacing detector Dz. This arrangement provides y the frequency diversity so essential to the faithful transmission of signals under varying coni tion may be applied to other and widely varied organizations without departing from the spirit lof the invention and the scope of the appended claims.

What is claimed is:

1. In a telegraph signaling system, the combination of means for producing currents of two frequencies in accordance with each marking signal and currents of two other frequencies inaccordance with each spacing signal, an amplifier for amplifying all of said currents, a limiter connected to said amplifier, the limiter acting to effectively suppress the weaker of the two currents corresponding to each marking signal as well as the weaker of the two currents corresponding to each spacing signal, means connected tosaid limiter for separating the different currents from each other, two detectors one for detecting the stronger of the two currents corresponding to eacli marking signal and the other for detecting the stronger of the two currents corresponding to each spacing signal, and means for reproducing the marking and spacing signals from said detected currents. v

2. In a telegraph system, the combination of means for producing currents of first and second frequencies in accordance with marking and spacing signals respectively, means for producing two currents which are modulation products oi the currents of said iirst and second frequencies respectively and transmitting said modulation products along with the currents of said ilrst and second frequencies. receiving means including means for obtaining currents corresponding to said irst frequencyand its modulation prod- -uct for each marking signal and currents corresponding to said second frequency and its modulation product for each spacing signal, a limiter i or effectively suppressing the weaker of the currents corresponding to'said marking and spacing signals, and means for detecting the stronger currents corresponding to each marking signal separate from the strongercurrents corresponding to each spacing signal.

3. Telegraph apparatus for receiving a plurality of currents of ditlerent frequencies corresponding to a marking signal and a plurality of other currents of different frequencies corresponding to a spacing signal, comprising a, limiter effectively accentuating the strongest of the .asoman cator coupled to said marking and spacing signals.

4. In a telegraph system in which a plurality of currents of diilerent frequencies are produced for each marking signal and a plurality of other currents of diilerent frequencies are produced for each spacing signal, the combination of an amnal, a second detector'for detecting the strongest currents corresponding to each marking signal and the strongest of the currents corresponding to each spacing signal while substantiallyk suppressing all of the other currents corresponding to said^`marking and spacing signals, two detectors coupled to said limiter for detecting the accentuated currents corresponding to a marking signal separately from the accentuated currents corresponding-to a spacing signal, and an indiof the currents correspondingto a spacing signal, means interposed between said ampllfierlimiter and said first and second detectors for sen lectively transmitting the currents corresponding to said marking andspacing signals, and a relay connected to both detectors for reproducing said detected marking and spacngslgnals.

5. In a telegraph system, the combination of means for producing marking and spacing'signais alternately, each marking signal and each spacing signal consisting of currents of a plurality of different frequencies, a limiter through which all oi said currents are transmitted, a plurality of filters for separating each current from all other currents, two detectors, one "detector being connected to all of the illters passing currents Vcorresponding to a marking signal and the other detector being connected to all of the filters passing currents corresponding to a spacing signal,'

said limiter acting to accentuate` the strongest of the currents corresponding to each marking signal as Well as the strongest of thev currents corresponding to each spacing signal while effectively'suppressing all of the weaker currents, and means controlled by both detectors for reproduc ing the marking and spacing signals. l v

HAROLD A. RHODES.

bota detectors for indicating. v 

